It is known that a continuous helical ribbon of thermoplastic material can be formed in an extrusion process. This can be done by applying a rotary motion to the extrusion die, the latter being provided with a slit orifice to produce the ribbon shape with the rotary motion of the die imparting the desired helical configuration. While this procedure is satisfactory for producing the intended helical thermoplastic material configuration, it has certain drawbacks. If the processing operation is run at high speed, i.e. relatively high ribbon line speed, there is likely to be imparted to the ribbon a whipping action tending to rotate the ribbon about its travel axis and with a whip spread of increasingly enlarging expanse which can lead to rupture of the ribbon with consequent process breakdown. Furthermore, it is difficult and expensive to construct an extrusion die of the rotating type due to the high order of material pressure therein (up to several thousand psi) without there being problems associated with leakage of the compressed thermoplastic resin from the extrusion chamber into bearing surfaces within the die.
It is also known that a flat ribbon of malleable or readily workable material, such as thermoplastic material can be passed between a pair of crossed rolls to impart a twist to the material. However, this procedure also imparts a rotary or rotational effect to the ribbon which at high speed can similarly result in creation of a whipping action leading to possible breakdown of the processing procedure. Of more consequence and if the twisted material were intended to be inserted into a tubular component of similar material, the rotary motion of the ribbon would interfere with effective combination of the two parts to produce a desired intended composite structured product.
Thus while it is known to produce a continuous ribbon of thermoplastic material which has a helical configuration, the prior art has not provided optimal method and apparatus for achieving this purpose.